Transmission Electron Microscopy of Solder Joints of Sn-Ag-Cu/Electroless Ni-P and Sn-Ag-Cu-Bi/Electroless Ni-P of an Electronic Device

Microstructures of joints between Pb-free solders (Sn-Ag-Cu and Sn-Ag-Cu-Bi) and an electroless Ni-8 mass%P were analyzed comprehensively using transmission electron microscopy(TEM). The specimens for TEM observation were prepared using a focused ion beam(FIB) technique. Columnar and spherical Kirkendall voids are formed at interfaces in the solder joints. TEM observation of a fractured joint reveals that these KV’s lead to degradation of the strength of the joints. Electron diffraction and EDX analysis show definite evidence for occurrence of η-(Cu, Ni)6Sn5 at the interface. Evolution of microstructure during thermal cycling in a temperature range between 253 and 453 K was also analyzed.